Voltage switch with regulated output current



March 16, 1965 D. WORTZMAN VOLTAGE SWITCH WITH REGULATED OUTPUT CURRENTINVENTOR DONALD WORTZMAN Filed. April 8, 1960 United States Patent3,174,054 VOLTAGE SWITCH WITH REGULATED OUTPUT CURRENT Donald Wortzman,New Paltz, N.Y., assignor to International Business MachinesCorporation, New York,

N.Y., a corporation of New York Filed Apr. 8, 1960, Ser. No. 20,853 7Claims. (Cl. 307-885) The present invention relates to electronicswitches and, more particularly, to a voltage switch for developing ananalog voltage proportional to a digital representation.

Analog-to-digital and digital-to-analog converters often employ avoltage-addition ladder network for developing an analog voltageproportional to a digital representation. There will be a resistor foreach order of the digital information having a value proportional to.theweight of the order. The analog voltage is developed by selectivelyapplying a reference potential or ground to the weighted resistors undercontrol of the digital information. The reference potential isattenuated different amounts through the weighted resistors and thencombined at the common output of the resistors to de- Velop the analogvoltage. Suitable switching means must be provided, responsive to thedigital representation, for selectively applying to the weightedresistors the reference voltage or ground.

Systems are presently being developed for performing arithmeticoperations by combining analog techniques with digital techniques. Thesesystems require switching means, responsive to digital information, forselectively applying to the weighted resistors of a voltage-additionladder network either ground potential or a reference voltage which hasbeen previously generated to represent a numerical quantity.

Voltage-addition converters of prior art systems have been inadequatefor several reasons. Since each of the resistors energized by thedigital information must provide a voltage proportional to the order ofthe digital information, the impedances required must be of differentvalues. This has necessitated the use of a specially de signed switchingmeans for each stage of the ladder network. The power supply used toprovide the reference voltage to all the stages of the ladder networkhas had to be an expensive, closely regulated supply. Converting the Nstage binary number to an analog voltage representation will require thereference voltage to be applied to N weighted resistors. The referencevoltage may be applied to 2 diflferent combinations of weightedresistors, requiring the reference voltage source to regulate for 2different load conditions.

In hybrid systems, utilizing analog and digital techniques for numericalcomputation, a particular digital to analog converter may be fed with areference voltage generated by a previous digital to analog converter.The reference voltage applied to a particular stage may be of a varyingnature. Difiiculties have arisen in prior systems for providing aswitching means adapted to accurately switch either a ground potentialor a reference voltage, which may vary from a maximum amount to zero tothe weighted resistors of the ladder network.

It is the object of the present invention to provide a new and improvedtransistor voltage switch which is selfregulating with respect to avoltage source.

It is an important object of the invention to provide a new and improvedvoltage switch suitable for use in voltage-addition ladder networks.

3,174,054 Patented Mar. 16, 1965 It is an additional object of thepresent invention to provide a new and improved voltage switch suitablefor switching a varying reference potential or ground potential to anoutput load.

It is also an object of the present invention to provide a transistorvoltage switch suitable for use in a voltageaddition ladder networkwherein the regulated reference voltage is not required to furnishsaturating current to the transistors.

It is an additional object of the present invention to provide atransistor voltage switch suitable for use in a voltage-addition laddernetwork wherein the current drain on the regulated voltage reference isa constant value regardless of the permutation arrangement of Weightedresistors connected to the reference voltage source.

It is a further object of the invention to provide a new and improvedvoltage switch which is relatively simple in construction andinexpensive to manufacture.

It is another object of the present invention to provide a new andimproved voltage switch suitable for several uses without requiringexpensive re-design.

In accordance with the invention, a voltage switch is provided whichcomprises a series connection of the emitter-collector paths of a firstand second transistor of opposite conductivity type. The output, towhich is applied either a reference voltage connected to the firsttransistor or ground connected to the second transistor, is taken fromthe electrical connection between the two transistors. A floating supplyis provided as part of a biasing means which normally causes the firsttransistor to be non-conductive and the second transistor conductive.The floating supply is suitable for supplying the necessary current tothe first transistor to place it in saturation. A voltage levelresponsive means including a third transistor, which forms part of thebiasing means, is provided for changing the conductivity state of thefirst and second transistors. The reference potential connected to thefirst transistor, and ground, connected to the second transistor arethen alternately applied through the respective low resistanceemitter-collector paths to the output. When the reference voltage is notconnected to the load through the first transistor, an equal load isprovided for the reference voltage by way of a resistor connectedbetween the reference voltage and the floating power supply.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

In the drawings:

FIG. 1 is a circuit diagram of the voltage switch in accordance with aparticular form of the present invention;

FIG. 2 is an incomplete circuit diagram of a voltageaddition laddernetwork employing the voltage switch of the present invention;

FIG. 3 is a circuit diagram of a voltage switch in accordance with amodified form of the invention;

Referring now to FIG. 1 of the drawing, a voltage switch thererepresented comprises a first transistor 10 and a second transistor 11.A series path is described from a terminal 12, to which is applied areference voltage, to ground through the emitter 13 and collector 14 oftransistor l0 and emitter l5 and collector 16 of transistor H. Thecollector 14 of transistor 19 and the emitter 15 of transistor 11 areconnected together at a junction point 17.

The output of the voltage switch is taken across a load resistor 18 atan output terminal 19. The base 29 of transistor and base 21 oftransistor 11 are connected together at a point 22. A source ofsaturating current for transistor 10 is provided through a pathincluding the base-collector junction of transistor 16, point 17, a pairof similarly poled asymmetrically conductive devices 23 and 24, resistor25, an unregulated voltage source 26, and the base electrode 2%) oftransistor it). An asymmetrically conductive device 27 is providedbetween ground and the junction of diode 24 and resistor 25. Normalbiasing is provided by a voltage level responsive transistor 28.Transistor 28 is connected to point 22 through its collector 29 andresistor 30. The negative potential applied to the emitter 31 oftransistor 23, and the positive potential applied through resistor 32 tothe base 33 of transistor 28 normally biases the transistor 28 to beconductive. Transistor 28 may be rendered nonconductive by theapplication of a negative voltage level at terminal 34. Circuit meansincluding a resistor 35 connects the terminal 12 to the base electrodeof both transistors 10 and 11 and the unregulated voltage source 26 atpoint 22.

Considering now the operation of the voltage switch, with transistor 28normally conducting in saturation, point 22 will be at a potential morenegative than ground. Current will be drawn from the base 21 oftransistor 11 causing transistor 11 to be saturated. Point 17 and theoutput at terminal 19 will be at ground potential through the lowresistance emitter-collector path of transistor 11. With transistor 28in saturation, current will also be drawn through forward biased diode27, resistor 25 and the unregulated voltage source 26. Current drawnthrough diode 27 will cause the junction of diode 27 and diode 24 to beslightly negative with respect to ground at point 17. The bias createdacross diodes 23 and 24 would be sufiicient to cause one to conduct, butwhen equally divided between the two, neither has sufiicient bias toconduct. In an actual embodiment of the invention, the bias needed tocause conduction, and the voltage drop across the diodes is about 0.3volt. If 0.3 volt is placed across diodes 23 and 24, each would receive0.15 volt, so that neither could conduct. An alternate method would beto replace diodes 23 and 24 with a single germanium diode and make diode27 a silicon diode. Germanium diodes require a greater bias than silicondiodes to cause conduction. As a result, no current is drawn away fromemitter 15 of transistor 11, insuring that junction 17 will bemaintained at ground potential.

A negative voltage level applied at terminal 34 will render transistor28 non-conductive. It should be recalled that a transistor startsconducting when either the base-collector or base-emitter junction isforward biased. 'vVhen transistor 28 is rendered non-conducting, point22, and therefore base 20, will rise in voltage. When compared with thecollector 14 potential as effected by the floating potential of battery26, the base-collector junction will be forward biased. The unregulatedvoltage source 26 will now provide saturating current for transistor 10causing it to conduct heavily. Diodes 23 and 24 will be forward biasedbecause point 17 will be clamped to the plus reference voltage throughthe low resistance emitter-collector path of transistor 10. As a result,the output terminal 19 across the load 18 now has applied to it the plusreference voltage and has been switched from ground potential to theplus reference voltage. With point 17 at the plus reference voltage, anddiodes 23 and 24 conducting heavily, diode 27 will be back biaseddrawing no current. Since point 22 which is applied to the base 21 oftransistor 11 is more positive than the plus reference voltage at point17, transistor 11 will be rendered nonconductive.

l Vhen transistor 10 is fully conductive, the plus reference voltagewill be connected to point 17 and an amount of current will be drawnfrom the reference depending 4 upon the size of the load resistor 18.Resistor 35 has been included between the reference voltage and point 22to draw an amount of current from the voltage reference, when transistor16 is non-conductive, equfl to the current drawn by the load resistor 18when transistor 1% is con-* ductive. As a result, the voltage referencewill be re quired to furnish the same amount of current independent ofthe conductivity state of transistor 10. Since the cur: rent drawn fromthe plus voltage reference is maintained at a constant value, a lessexpensive regulated voltage source may be used. Without this feature,the reference voltage would be required to furnish several values ofcur-- rent especially when the voltage switch is used in a voltageaddition ladder network with many combinations of output loads on thevoltage reference. No matter how many voltage switches of the presentinvention are required in a voltage-addition network, each stageprovided will always draw the same amount of current from the voltagereference no matter what the combination of voltage switches have beenenergized. I

FIG. 2 shows a partial arrangement of a voltage-addition ladder networkemploying the present invention. The resistors 1% correspond to the loadresistor 18 shown in PEG. 1. The resistors 18 will be weighted tocorrespond to the weight of its respective order of digital information.The reference voltage is applied at terminal 12 of the ladder network.The analog voltage which is to assume a value proportional to thedigital representation is developed at terminal 36.

The voltage switch shown in FIG. 1 must be modified if it is to be usedin a hybrid system foraccomplishing arithmetical operations utilizingboth analog and digital techniques. In the hybrid systems, a firstdigital-to analog converter may be feeding an analog voltage to a succd=ing digital-to-analog converter. It is possible that a pre cedingconverter may be feeding an analog voltage of zero volts to a succeedingstage to represent a digital input of zero. The voltage switch shown inFIG. 1 would not be capable of accurately switching the point 17 fromground to the reference voltage which might be zero volts. Whentransistor 10 is saturated, clampingpoint 17 to the reference voltage,diodes 23 and 24 are fully conductive. For normal positive values of thereference voltage, diode 27 Will be back biased. As the referencevoltage approaches a value just slightly more positive than ground, thebias across diode 27 Will approach a value which will cause diode 27 tostart conducting. The output potential at point 17 will therefore beclamped to a value equal to the slight drop in potential across diode27.

FIG. 3 shows a modified arrangement of the voltage switch of FIG. 1 forallowing the accurate switching of a reference voltage of zero volts topoint 17. Identical components of FIG. 1 and FIG. 3 are given the samenumeral designations. The conductivity state of diode 27 is moreaccurately controlled by including a transistor 40 in the circuit. Inthe normal state, transistor 40 is non conductive and transistor 28 isconductive as in FIG. 1. The collector 41 of transistor 40 is connectedto a voltage divider network consisting of diode 42 and resistor 43. Theplate of diode 27 is connected to the junction of the collector 41 andthe voltage divider network. When transistor 40 is non-conductive theplate of diode 27 will be held at approximately ground potentialcorresponding to the ground shown connected to diode 27 in FIG. 1.Transistor 11 will be normally conductive, as before, clamping the point17 at ground potential. A positive rise in potential applied at terminal34 will cause transistor 4t to become conductive. The plate of diode 27will now be placed at a potential more negative than ground through thelow resistance emitter-collector path of transistor 40. The negativepotential at the plate of diode 27 will insure that diode 27 will alwaysbe back biased and never conductive even though point 17, which is nowclamped to the reference voltage, should become zero volts. The basicvoltage switch shown in FIG. 1 is now puv w capable of use in a hybridarithmetic system where the reference voltage applied to the voltageswitches might approach a value of zero.

While a voltage switch for use with a positive reference voltage isshown, it will be apparent to those skilled in the art that a negativereference voltage switch may be made from this invention by reversingall voltages, diodes and the conductivity type of the transistors.

It is apparent from the description that use of an expensive highlyregulated voltage source is not necessary with the present invention.The reference voltage may be generated, for instance, from a simple andinexpensive Zener diode system. Since the voltage switch is providedwith saturating current from a source other than the reference source,high current output will not be required from the reference source. Thesmall amount of current required from the reference source has beenshown to be a constant value no matter what combination of voltageswitches has been energized by digital'information inputs. It has alsobeen shown that the voltage switch of the present invention may be ofone simple design regardless of the value of the reference voltage orthe weighted load resistors.

While the applicant does not wish to be limited to any particular set ofcircuit constants, the following constants have proved to be useful in avoltage switch of the type represented in FIG. 1:

Resistor25 1.1K Resistor 30 698 Resistor 32 n 22K Diode 23 IN461 Diode24 IN461 Diode 27 IN461 Transistor it) IBM 083 Transistor 11 IBM 033Transistor 28 IBM 065 Battery 26 6volts +V ref 24 volts +E 6volts E6volts While the invention has been particularly shown and describedwith reference to preferred embodiments thereof, it Will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention.

I claim:

1. A voltage switch for accurately applying a first voltage or a secondvoltage to the first terminal of an output load the second terminal ofwhich is connected to an output reference potential, comprising incombination: a first and second transistor, each of said transistorshaving base, collector and emitter electrodes, the emittercollector pathof said transistors being in series between the first and secondvoltages, the collector of one of said transistors and the emitter ofthe other of said transistors being connected to the first terminal ofsaid load at a common point, a source of biasing reference potential,biasing means including an unregulated voltage source connected betweensaid biasing reference potential and the base electrode of both of saidtransistors for biasing said second transistor to be normally conductiveand said first transistor normally non-conductive, input circuit meansconnected to said biasing means, said biasing means responsive to achange in voltage level at said input means for changing theconductivity state of both said transistors, impedance means having animpedance equal to the impedance of the output load connecting the firstvoltage to said base electrodes and said unregulated voltage, and meansconnecting said common point to a point between said biasing referencepotential and said unregulated voltage source, controlled by saidbiasing means for blocking the flow of current from the common point ofsaid transis- 6 tors through said unregulated voltage source when saidsecond transistor is in its normally conductive state.

2. A voltage switch for accurately applying a first voltage or a secondvoltage to the first terminal of an output load the second terminal ofwhich is connected to an output reference potential, comprising incombination: first and second transistors, each of said transistorshaving base, collector and emitter electrodes, the emittercollector pathof said transistors being in series between the first and secondvoltages, the collector of said first transistor and the emitter of saidsecond transistor being connected to the first terminal of said load ata common point, a source of biasing reference potential, biasing meansincluding an unregulated voltage source connected between said biasingreference potential and the base electrode of both of said transistorsfor biasing said second transistor to be normally conductive and saidfirst transistor normally non-conductive, input circuit means connectedto said biasing means, said biasing means responsive to a change involtage level at said input means for changing the conductivity state ofboth said transistors, circuit means connecting the first voltage tosaid base electrodes and said unregulated voltage, and means connectingsaid common point to a point between said biasing reference potentialand said unregulated voltage source, controlled by said biasing meansfor passing the flow of current from the collector to the base of saidfirst transistor through said unregulated voltage source when changed toits conductive state.

3. A voltage switch for accurately applying a first voltage or a secondvoltage to the first terminal of an output load the second terminal ofwhich is connected to an output reference potential, comprising incombination: first and second transistors, each of said transistorshaving base, collector and emitter electrodes, the emitter collectorpath of said transistors being in series between the first and secondvoltages, the collector of said first transistor and the emitter of saidsecond transistor being connected to the first terminal of said load, asource of biasing reference potential, biasing means including anunregulated voltage source connected between said biasing referencepotential and the base electrode of both of said transistors for biasingsaid second transistor to be normally conductive and said firsttransistor normally non-conductive, input circuit means connected tosaid biasing means, said biasing means responsive to a change in voltagelevel at said input means for changing the conductivity state of bothsaid transistors, circuit means connecting the first voltage to saidbase electrodes and said unregulated voltage, and a pair of seriesconnected, similarly poled, asymmetrically conductive devices connectedbetween said unregulated voltage source and the collector and emitter ofsaid first and second transistors respectively, controlled by saidbiasing means, said pair of asymmetrically conductive devices adapted topass saturating current for said first transistor through saidunregulated voltage source when said first transistor is conductive andblock the fiow of current to said unregulated voltage source when saidsecond transistor is conductive.

4. A voltage switch in accordance with claim 3 wherein said biasingmeans includes an asymmetrically conductive device connected betweensaid source of biasing referene potential and said unregulated voltagesource poled to be non-conductive when said first transistor isconductive and conductive when said second transistor is conductive.

5. A voltage switch in accordance with claim 4 wherein said biasingmeans includes a third normally nonconductive transistor responsive tothe change in voltage level at said input means adapted to apply avoltage to said asymmetrically conductive device more negative than thefirst voltage when said first transistor is conductive and a voltageequal to the second voltage when said second transistor is conductive.

6. A voltage switch in accordance with claim 3 wherein said circuitmeans is an impedance which causes a current drain on the first voltageequal to the current drain caused by the load, independent of theconductivity state of said first transistor.

7. A voltage switch in accordance with claim 3 Wherein said unregulatedvoltage source provides the saturating current for said firsttransistor.

References Cited by the Examiner UNITED STATES PATENTS 2,851,604 9/58Clapper 30788.5 2,864,007 12/58 Clapper 30788.5 2,880,332 3/59 Wanlass307-885 2,885,573 5/59 Clapper 30788.5

JOHN W. HUCKERT, Primary Examiner.

GEORGE N. WESTBY, HERMAN K. SAALBACH,

ARTHUR GAUSS, Examiners.

1. A VOLTAGE SWITCH FOR ACCURATELY APPLYING A FIRST VOLTAGE OR A SECONDVOLTAGE TO THE FIRST TERMINAL OF AN OUTPUT LOAD THE SECOND TERMINAL OFWHICH IS CONNECTED TO AN OUTPUT REFERENCE POTENTIAL, COMPRISING INCOMBINATION; A FIRST AND SECOND TRANSISTOR, EACH OF SAID TRANSISTORSHAVING BASE, COLLECTOR AND EMITTER ELECTRODES, THE EMITTERCOLLECTOR PATHOF SAID TRANSISTORS BEING IN SERIES BETWEEN THE FIRST AND SECONDVOLTAGES, THE COLLECTOR OF ONE OF SAID TRANSISTORS AND THE EMITTER OFTHE OTHER OF SAID TRANSISTORS BEING CONNECTED TO THE FIRST TERMINAL OFSAID LOAD AT A COMMON POINT, A SOURCE OF BIASING REFERENCE POTENTIAL,BIASING MEANS INCLUDING AN UNREGULATED VOLTAGE SOURCE CONNECTED BETWEENSAID BIASING REFERENCE POTENTIAL AND THE BASE ELECTRODE OF BOTH OF SAIDTRANSISTORS FOR BIASING SAID SECOND TRANSISTOR TO BE NORMALLY CONDUCTIVEAND SAID FIRST TRANSISTOR NORMALLY NON-CONDUCTIVE, INPUT CIRCUIT MEANSCONNECTED TO SAID BIASING MEANS, SAID BIASING MEANS RESPONSIVE TO ACHANGE IN VOLTAGE LEVEL AT SAID INPUT MEANS FOR CHANGING THECONDUCTIVITY STATE OF BOTH SAID TRANSISTORS, IMPEDANCE MEANS HAVING ANIMPEDANCE EQUAL TO THE IMPEDANCE OF THE OUTPUT LOAD CONNECTING THE FIRSTVOLTAGE TO SAID BASE ELECTRODES AND SAID UNREGULATED VOLTAGE, AND MEANSCONNECTING SAID COMMON POINT TO A POINT BETWEEN SAID BIASING REFERENCEPOTENTIAL AND SAID UNREGULATED VOLTAGE SOURCE, CONTROLLED BY SAIDBIASING MEANS FOR BLOCKING THE FLOW OF CURRENT FROM THE COMMON POINT OFSAID TRANSISTORS THROUGH SAID UNREGULATED VOLTAGE SOURCE WHEN SAIDSECOND TRANSISTOR IS IN ITS NORMALLY CONDUCTIVE STATE.